Image wave suppressor



Patented June 9, 1942 1 i i IMAGE WAVE SUPPRESSOR Fulton Cutting, New York, N. Y., assignor of onehalf to Colonial Radio Corporation, Bu'alo, N. Y., and one-half to The F. W. Sickles C'ompany, Springfield, Mass.

Application May 4, 1940, Serial No. 333,327

7 Claims. ,(Cl. Z50-20) duced by the local oscillator, and this gives rise l' to oscillations of a frequency which may be either th'e sum of or the difference between the incoming frequency and that of the local oscillator.

through the intermediate frequency amplifier are then amplified.

For example, if the intermediate frequency amplifier is tuned to 400 kilocycles and a 550 kilocycle signal is being received, and the local oscillator has, when this particular signal is being received, a frequency of 950 kilocycles, then there will be produced a beat frequency of 400 kilocycles. However, if there vis'also an incoming signal of 1350 kilocycles at the same time, a new oscillation will be produced by the heterodyne action between the local oscillation and the 1350 kilocycle signal, and this new oscillation will likewise have a frequency of 400 kilocycles and will accordingly pass through the intermediate frequency amplifier, resulting in interference and cross-talk with the desired signal. The frequency of the undesired signal is termed the image frequency, and it is equal to the frequency of the desired signal plus twice the intermediate frequency.

It is usual to provide a signal selecting circuit in advance of the first detector, and this circuit is ordinarily arranged to be tuned to the frequency of the desired signal. However, even when such a circuit is used, image interference may still be encountered and it has been proposed in the past to eliminate or reduce this image interference by providing preselecting circuits and the like, and also by providing one or more wave traps associated with the antenna and variably tuned.

The employment of wave traps for image suppression increases the cost of the receiver, since it is necessary to provide an additional coil and an additional condenser, one or the other of which is usually variable. The provision of such parts increases the cost of the receiver, from the standpoint of material and parts 'employed and also the labor of assembly.

Only those oscillations thereby produced I which have a frequency enabling them to pass I have discovered that it is possible to provide a simple tuning circuit with only one variable tuning reactance which acts simultaneously as a selector of desired frequencies and as an image suppressor; that is to say, the variation of the variable reactance causes the circuit to select the desired signal, at the same time to present `an extremely high impedance to the image signal, and I have found that circuits according to my invention produce signal to image ratios as high as, and in many cases considerably higher than, the best signal to image ratios produced by conventional wave trap circuits. As an example, employing the single tuned circuit Aof my invention, I have been able to obtain signal to image ratios as high as 70,000.

Among the objects of my invention are:

To provide an image suppression circuit which l does not require separate wave trap or preselector circuits;

To provide a circuit in which the image suppression is automatically performed by the main tuning circuit.

Still another object of my invention is to provide a simple signal selecting circuit which will deliver signal to image ratios as high orhigher than those obtained from conventional wave trap or preselector image Suppressors.

Still other objects of my invention will be apparent from the specification,

In this application I have particularly pointed out and distinctly claimed the part, improvement or combination which I claim as my invention or discovery and II have explained the principles thereof and the best mode in which I have contemplated applying those principles so as to distinguish my'invention from other inventions.

In the drawing- Fig. 1 is a circuit diagram of one form of circuit in accordance with my invention;

Fig 2 is a circuit diagram of one modification of my invention; and

Fig. 3 is a circuit diagram of a further modication of my invention.

In the practice of my invention, for some purposes I prefer to employ what is known as permeability tuning, although this invention is not limited thereto. If such permeability tuning is employed, the variation in resonant frequency is obtained, not by rotation of a variable condenser,

`but by movement of a core of magnetic material inthe field of an inductance. Such cores are satisfactorily formed of iron dust mixed with a suitable insulating and binding material and pressed or otherwise processed to form what appears as a solid bar.

Since they are easily obtainable on the market, and their construction is per se, not a part of my invention, they are not described in detail.

If such a core is moved axially into and out of a solenoid in a resonant circuit, there is observed a variation in the inductance of the solenoid, and a variation of the resonant frequency of the circuit in which said solenoid is connected, this change of resonant frequency .being due, to a very considerable extent, to the variation in the value of the inductance of the solenoid with the position of the core, and probably also, in some arrangements, to some extent at least, to the variation of the capacity of the circuit resulting from change of position of the core.

Referring more particularly to Fig. 1, I represents an antenna of any suitable type Whichrmay be connected to ground through coupling condenser 2. Currents produced in the antenna I by incoming signals will flow to some extent through inductance 3, shunted by condenser 5, and the voltage thereby produced may be supplied to the grid of the first tube l, which may be connected to ground through condenser 6.

The antenna side of coil 3 may be connected through resistance II, having a high resistance, to ground or to the automatic volume control voltage source, if such is present, thereby providing a direct current grid return path. The cathode of tube I may be grounded through resistance I2, shunted by condenser I3, or directly grounded if desired.

Since my invention is not concerned with the construction of the superheterodyne receiver beyond this point, and the same is Well known in the art, only the first tube 'I is indicated.

The desired signals may be selected by movement of the core 4 into and out of the field of coil 3, and at the same time the frequency of the local oscillator is varied in such a manner that the signal selected in the antenna circuit by movement of the coil 4 is heterodyned to produce oscillations of the desired intermediate frequency.

In adjusting the circuit for operation, the condenser 5 may be omitted and the desired number of turns in inductance 3 provided so that movement of the core 4 causes the circuit to tune over the range desired, in this instance from 500 to 1500 kilocycles, and the tuning circuit formed by inductance 3 and its associated elements, herein called the signal tuning circuit, is preferably tracked with the oscillator tuner by trimmer condenser 6 so that the receiver may be operated by a single control as indicated by the connected arrows in the drawing. The circuit will now operate as a conventional superheterodyne receiver and image interference may be experienced.

If now condenser 5 be connected across coil 3 and chosen of the proper value, a very great improvement in signal toimage ratio is noticed. In choosing the value of condenser 5, it is picked of such magnitude as to cause the inductance 3 to resonate with condenser 5 at the image frequency (which, as stated, is equal to the desired signal frequency, plus twice the intermediate frequency) when the tuning circuit is tuned to a desired signal frequency.

The connection of this condenser, which will ordinarily be quite small, for instance of the order of two to twelve M. M. F., may slightly disturb the tracking -of the main tuning circuit with the oscillator tuner, in which case it may be necessary to retrim the main tuning circuit by variation of the trimmer condenser 6, but it will be found that when the operation is completed the entire input circuit, as a Whole, is resonant to the desired signal frequency; whereas, inductance 3 and condenser 5, forming a closed loop circuit, are resonant to the image-producing signal and cause extreme attenuation thereof while not interfering with the selection by resonance of the desired signal. I have found that movement of the core 4 into and out of the coil 3 varies the tuning of the entire circuit and also the tuning of the image suppressor 3-5 in a satisfactory approximation to the desired manner.

In order for the signal or main tuning circuit to vary from 500 to 1500 kilocycles, i. e., 3 to 1 ratio, the inductance of that circuit must vary in a 9 to 1 ratio. It will be noted, however, that while the resonant circuit varies from 500 to 1500 kilocycles, the image frequency varies from 1460 to 2510 kilocycles when the intermediate frequency is 455 kilocycles. In this case the ratio of frequency change is 1.72 to 1 and the change of inductance required to make the image circuit track exactly should be in the square of this ratio, or 2.96-1.

From this it will be seen that if the main tuning circuit is tuned to the lowest broadcast fre- `quency, i. e., 550 kilocycles, and the image circuit '3 5 then tuned, by varying condenser 5, to the 'image frequency, then as the main tuning circuit frequency is increased by movement of the core 4 out of the inductance, the reduction of inductance vin the image trap proceeds too fast,

' because, ordinarily, the most harmful image interference is obtained when the main tuning circuit is tuned to the 10W frequency end of the broadcast band. However, in case this departure from the theoretical value of the image trap is desired to be reduced, it may be done by utilizing the circuits of Fig. 2 or Fig. 3, in which like reference numerals indicate like parts, as in Fig. 1.

In Fig. 2 I may provide an additional inductance 8 in series with inductance 3 and condenser 5, and in this instance inductance 8 is so arranged that the movement of the core 4 has substantially no effect on the value of inductance 8. Consequently, as core 4 is removed from inductance 3, the total inductance of the image trap does not change in the ratio of 9 to 1, but in a somewhat smaller ratio, and by choosing the value of inductance 8, the resonant frequency of the image trap may be brought closer to its required value.- This operation does not substantially affect ythe 9 to 1 variation of inductance of coil 3, which performs the main tuning function, because at the signal frequency, the impedance of inductance 8 and condenser 5 in series is still -too high for this circuit to operate as an effective shunt around coil 3.

By choice of the proper valuel of inductance 8 related to that of inductance 3, together with condenser 5, it is possible to cause the image trap to be in resonancel at the image frequency, both at the low frequency end and thehigh frequency endof the broadcast band, and if this is done, any departure of the resonantfrequency of the image trap from the image frequency, which may occur at intermediate points, is not likely to be serious. In this connection it should be noted that inductance 8 may or may not be coupled with inductance 3.

A still further arrangement is shown in Fig. 3, wherein inductance 8 is so arranged with reference to inductance 3 that as the core 4 moves out of the field of coil 3 it moves into the field of coil 8, This may be conveniently accomplished by winding inductances 3 and 8 on the same tubular form, but leaving a space of the desired length between them so that as the core is withdrawn axially from coil 3, it moves into coil 8.

With this arrangement of coils 3 and 8 there will be usually some coupling between these coils, and the nature of this coupling, i. e., aiding or opposing, may be chosen to produce the best results for particular conditions.

It may be noted that under certain conditions the value of condenser 2 may be zero; that is, that physical condenser may be omitted. One instance of this is in automobile radio installations where the running-boards are used as an aerial. In this instance, the antenna capacity, which is in parallel with condenser 2 is so large, comparatively, that condenser 2 may be omitted.

Condenser 2 may be omitted with an aerial of small capacity also, such as a lishpole In this instance condenser 6 may be adjusted to compensate for the variation in antenna capacity.

While I have shown and described certain preferred embodiments of my invention, it will be apparent that modifications and changes may be made without departing from the spirit and scope of my invention, as will be clear to those skilled in the art.

I claim:

1. In a superheterodyne receiver, in combination, inductance and capacity elements associated together to form a resonant circuit, tunable as a whole by variation of said inductance to select desired signal frequencies, and a circuit in shunt with said inductance, said shunt circuit comprising inductance and capacity, the circuit formed by said inductance and the elements'in shunt therewith being parallel tuned to the image of the desired signal frequency,

2. In a superheterodyne receiver, in combination, inductance and capacity elements associated together to form a resonant circuit, tunable as a whole by variation of said inductance to select desired signal frequencies, a circuit in shunt with said inductance, said shunt circuit comprising inductance and capacity, a magnetic core movable into and out of the field of said first inductance to vary the value thereof, the circuit formed by said inductance and said shunt circuit being maintained parallel resonant to the image of the selected signal frequency.

3. In a superheterodyne receiver, in combination, inductance and capacity elements associated together to form ar resonant circuit, tunable as a whole by variation of said inductance to select the desired signal frequencies, a shunt circuit connected around said inductance, said shunt circuit comprising inductance and capacity, a movable core of magnetic material movable into the field of both said inductances to tune said circuit as a whole to the desired signal frequency and to simultaneously tune said inductance and said shunt circuit as a parallel resonant circuit to the image of said selected signal frequency.

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4. In a superheterodyne receiver, in combination, inductance and capacity elements asso ciated together to form a resonant circuit, tunable as a whole by variation of said inductance to select the desired signal frequencies, a shunt circuit connected around said inductance, said shunt circuit comprising inductance and capacity, a movable core of magnetic material movable into the field of both of said inductances to tune said circuit as a whole to the desired signal frequency and to simultaneously tune said inductance and said shunt circuit as a parallel resonant circuit to the image of the selected signal, said inductances and said core being so arranged that as said core moves out of the iield of one of said inductances, it moves into the field of the other.

5. In a superheterodyne radio receiving circuit having an intermediate frequency, a thermionic relay having a control grid, a selector circuit comprising a first inductance coil, a first capacitor and a second inductance coil in series, the last two being in parallel with the first inductance coil, a second capacitor in series with the iirst inductance coil at one end, an impedance in series with the first inductance coil at the other end, the opposite ends of the impedance and second capacitor being grounded, means for im- Ypressing a received signal across the terminals of the impedance, the second capacitor and the first inductance coil being connected to the grid, and a ferro-magnetic core movable in the first inductance coil for varying its inductance to tune the circuit as a whole to a desired frequency and to tune the image trap composed of the first inductance coil, the second inductance coil and the iirst capacitor to a frequency substantially equal to the desired frequency plus twice the intermediate frequency.

6. A preselector system for a superheterodyne radio receiver, including a variable inductance device comprising a first inductance coil and a movable ferromagnetic core, said core having surface conductivity and being effectively grounded, first and second capacitors associated with said inductance device to secure resonance within a first range of frequencies by movement of said core, and a series-connected second inductance coil and third capacitor in shunt with said inductance device to secure resonance within a second range of frequencies by movement of said core, said capacitors being so related to said second inductance coil and to the capacitance between said iirst inductance coil and said core that any frequency of said second range is higher than the corresponding frequency of said first range by a substantially constant amount throughout said core movement.

7. In a superheterodyne receiver, in combination, a first inductance and capacity elements associated together to forma resonant circuit, tunable as a whole by variation of said inductance to select the desired signal frequencies, a shunt circuit comprising capacity anda second inductance connected around said first inductance, a movable core of magnetic material movable into the field of said first inductance only, to tune said circuit as a whole to the desired signal frequency and to simultaneously tune said inductance and said shunt circuit as a parallel resonant circuit to the image of said desired signal frequency.

FULTON CUTTING. 

